Zinc oxide is a II-VI semiconductor with considerable potential for optoelectronic applications in the UV spectrum due to its wide direct band gap (3.4 eV at 300 K), high exciton binding energy (60 meV), and excellent radiation hardness.
Zinc oxide can be grown using a variety of techniques, and bulk single crystal wafers are available from a number of different manufacturers. Bulk growth techniques used by manufacturers include seeded chemical vapor growth, melt growth and hydrothermal growth. Of these, hydrothermal growth results in material of significantly lower carrier concentration and higher resistivity compared to other growth techniques
Like gallium nitride, zinc oxide crystallizes into the wurtzite structure with a c:a ratio lower than that for perfectly hexagonally close-packed atoms (c:a=1.6020 for zinc oxide and 1.6333 for hcp). This, together with a lack of inversion symmetry and the ionic nature of the Zn—O bond, produces a net dipole moment along the c axis of the unit cell.
When zinc oxide is cleaved normal to the c axis it forms a Zn-polar (0001) face whose outer plane consists entirely of zinc atoms and an opposite O-polar (000 1) face consisting of an outer plane of oxygen atoms. Zinc oxide has a large spontaneous polarization (−0.057 Cm−2) along the c axis due to the net effect of internal dipole moments.
Schottky contacts to zinc oxide were first reported in 1965 on vacuum cleaved n-type surfaces (see C. A. Mead, Phys Lett. 18 218 (1965)). Since then an increasing amount of research has been carried out using gold, silver, platinum and palladium as the Schottky metals. The majority of the reported barrier heights are in the range 0.6-0.8 eV, with the quality of the zinc oxide material and the surface treatment seemingly more important than the metal used (see A. Y. Polyakov et al., Appl. Phys Lett. 83 1575 (2003)).
It is well known that the condition of the semiconductor surface is critical to the performance of Schottky contacts, with many possible influences such as surface morphology, surface states, metal-surface chemical reactions, hydrocarbon and hydroxide surface contamination, oxygen chemisorption, and subsurface defects. Another potential variable is the polarity of the zinc oxide surface. Nonzero net dipole moments occur perpendicular to the Zn-polar (0001) and O-polar (000 1) surfaces, which may lead to differences in the interface structure of Schottky contacts and may also affect some of the surface factors listed above.
Coppa et al. (J. Appl. Phys 97 103517 (2005)) reported no significant surface polarity related effects on the improvement of the rectifying properties of gold contacts on the (0001) and the (000 1) surfaces of bulk zinc oxide wafers following remote oxygen/helium plasma cleaning at 525-550° C. This improvement was attributed to the removal of hydrocarbon and hydroxide contamination and also the surface chemisorption of oxygen species following cooling in the unignited plasma ambient. Mosbacker et al. (Appl. Phys. Lett. 87 012102 (2005)) reported a conversion from ohmic to rectifying behavior of gold contacts on bulk zinc oxide following remote room temperature oxygen/helium plasma treatment which was almost identical on the Zn-polar and O-polar faces. This was again attributed to removal of a carbon and hydroxide accumulation layer from the zinc oxide surface plus the removal of near surface defects and donors.
It is an object of the present invention to provide an improved Schottky-like or ohmic contact; and/or to at least provide the public with a useful choice.
Other objects of the invention may become apparent from the following description which is given by way of example only.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date.